1. Field of the Invention
The present invention relates to a tracking servomechanism for a disc drive apparatus having a playback head and a recording head attached individually to a rotary support arm.
2. Description of the Prior Art
The conventional head displacement means known heretofore for use in a disc drive apparatus are generally classified into a linear type which moves a support arm linearly to displace a head in the radial direction of a disc, and a rotary type which rotates a support arm to displace a head in the radial direction of a disc. To realize a compact and lightweight structure of the drive apparatus, the rotary type is considered to be preferable.
Meanwhile, a magnetoresistive type head capable of exhibiting excellent playback sensitivity characteristics even in a narrow track width has been developed to meet the requirement of achieving a higher track density. However, the magnetoresistive type head has no recording function. Therefore, when such magnetoresistive type head is employed, it is necessary to provide a recording head in addition thereto.
Under the condition mentioned, it is still possible to produce a high density recording/reproducing disc drive apparatus of a compact and lightweight structure by using a rotary support arm and attaching a playback head and a recording head individually to such support arm.
FIG. 1 schematically illustrates the structure of a conventional disc drive apparatus equipped with a rotary actuator, in which a base end of a support arm 1 is held rotatably by a rotary shaft 2. This shaft 2 is rotated by a driving force of an unshown motor to control the position of the support arm 1. A slider 3 is attached to a distal end of the support arm 1, and a magnetoresistive type playback head Hp and a coil type recording head Hr are secured to the slider 3 as illustrated in FIG. 2. The playback head Hp and the recording head Hr are disposed on a center line Cl of the support arm 1 while being spaced apart by a predetermined distance from each other, in such a manner that the recording head Hr is set at a front position while the playback head Hp is set at a rear position. The playback head Hp and the recording head Hr are so arranged as to be both in an exact tracking or "just-tracking" state at an innermost servo signal track Ti on a disc 4. Therefore at any other track, the recording head Hr is placed in an off-tracking state when the playback head Hp is at a "just-tracking" position.
In such known disc drive apparatus equipped with a rotary actuator as shown in FIG. 1, the angle formed by the slider 3 and the tangential direction C to each track is changed in accordance with the track position. FIG. 1 represents an exemplary case where a directional coincidence is attained therebetween at the innermost track position Ti. The above angle gradually increases in accordance with the displacement of the slider 3 toward the periphery of the disc and finally reaches a maximum value .theta. at the outermost track position To.
In an exemplary case of FIG. 2 where the recording head Hr and the playback head Hp are so disposed on the slider 3 that the respective centers thereof are positioned exactly at the center line C2 of the innermost track Ti, when the rotary arm is controlled for aligning the center of the playback head Hp with the center line of any other track, the center of the recording head Hr comes to be positioned at a point deviated from the center line C2 of the track. When a tracking servo control is executed while a tracking servo signal prerecorded on the disc is read out therefrom by the playback head Hp, the center of the playback head Hp can be exactly positioned with respect to the track center, but there exists a positional deviation between the center of the recording head Hr and any other track center except the innermost track. Consequently, if a recording mode is executed in a state where merely the playback head Hp alone is aligned with the track center line as shown in FIG. 3, a newly recorded data track is formed with a positional deviation from the preceding data track. And when the signal recorded in this manner is reproduced, there arises a problem that some noise or read error is induced since the playback head Hp is so controlled that the center is continuously positioned on the track center line C2 while the recording head Hr scans the off-track position of the data signal at any track other than the innermost track Ti.
There is known a discrete track disc where servo tracks and data tracks are formed alternately in the circumferential direction on a magnetic disc, and both tracks are composed of a magnetic layer while any other region is composed of a nonmagnetic layer. In such discrete track disc, the width of the data track is smaller than the width of the recording head as shown in FIG. 4, so that a signal recording operation can be performed without causing any exudation or incomplete erasure in a recording mode.
However, even by the use of such a discrete track disc, when the off-track deviation becomes so great that the end of the recording head Hr scans the data track, some exudation or incomplete erasure is caused in the recording mode to eventually lose the merit of using a discrete track disc.